Method and device for providing a layer of coating material on the inner side of a display window for a color display tube
The invention relates to a method of providing a layer of a coating material on the inner side of a bowl-shaped rectangular display window for a color display tube, in which method a suspension of the coating material is provided on the inner side while the display window is horizontally arranged with its axis of symmetry oriented transversely to the inner side, whereafter the display window is rotated around its axis of symmetry for some time so as to distribute the suspension homogeneously on the inner side, said axis of symmetry being tilted until it is oriented substantially horizontally, during which rotation a part of the suspension provided is drained off the display window near its corners where it is collected in collection cups which are present near the corners during the rotation, and whereafter the suspension which is still present on the inner side of the display window is dried by means of thermal radiation. The invention also relates to a device for performing such a method.
A plurality of layers of a coating material is provided one upon the other on the inner side of a display window of a color display tube. In a color display tube, not only layers of red, green and blue fluorescent material are used but often also layers of a material transmitting only light of a given color or a material transmitting no light at all. After each layer has been provided, a pattern is formed therein. Each provided layer, to which photosensitive components are added specially for this purpose, is exposed in accordance with the pattern to be formed, whereafter the unexposed parts are dissolved in a developer. In display tubes for use in computer monitors, patterns of juxtaposed red, green and blue fluorescent dots are formed in the layers of fluorescent material, and patterns of juxtaposed red, green and blue fluorescent stripes are formed in display tubes for use in television apparatuses. A black matrixxe2x80x94a black layer with windows at the area of the fluorescent dots or stripesxe2x80x94may be formed in the layer of opaque material so that the contrast of images to be displayed can be enhanced. Patterns of color filters may be formed in the layers of a material transmitting only light of a given color, so that a better color rendition can be obtained.
During provision of the suspension, in which the display window may be stationary or rotate about its axis of symmetry oriented transversely to the inner side, this suspension is provided in excess. During the rotation, the suspension is homogeneously distributed on the inner side of the display window. This is effected by means of centrifugal forces and also by gravitational force when the axis of symmetry directed transversely to the inner side of the display window is in a horizontal position. An excess of suspension is drained off the display window near its corners. In these corners, the suspension is collected in collection cups which are present near the corners during the rotation, whereafter it may be used for coating other display windows. The suspension which is still present on the inner side of the display window is dried by means of thermal radiation, after which the desired coating is obtained.
A method of the type described in the opening paragraph is known from NLA-6 601 802, in which method the part of the suspension which is drained off the display window near its corners during the rotation is collected in closed collection cups. After a layer has been provided, the collection cups are emptied.
When a layer is provided by means of this method on a pattern which is already present on the inner side of the display window, a layer which does not have a homogeneous thickness may be formed. A layer may then be formed with a pattern of thickness differences caused by interaction between the suspension moving during the rotation with the pattern which is already present. This pattern of thickness differences may be visible when displaying images on display tubes having such display windows. In display tubes having display windows with a pattern of juxtaposed fluorescent dots, this is visible in the form of a 60xc2x0 cross, and in display tubes having display windows with a pattern of juxtaposed fluorescent stripes, this is visible in the form of a north-south line. Such image errors are unwanted.
It is an object of the invention to provide a method allowing manufacture of a display window in which the occurrence of these image errors is prevented when images are displayed on display tubes equipped with such a display window. To this end, the method is characterized in that, during the rotation, the part of the suspension collected in the collection cups near the corners of the display window is further drained from these cups to a co-rotating central collection cup.
Said image errors, 60xc2x0 cross and north-south line, may be suppressed by performing the rotation at a low velocity during provision of the layer of coating material. However, when the velocity decreases, there is the risk in the known method that the suspension in the collection cups drips onto the provided layer, particularly when the axis of symmetry oriented transversely to the display window is in a horizontal position at the end of the rotation. Such drops on the layer may render a display window unusable. Due to the measure according to the invention, it is prevented that suspension drips from the collection cups onto the display window during the rotation so that the rotation can be performed at such a low velocity that layers can be provided one on top of the other without said patterns of thickness differences being formed. Said image errors, 60xc2x0 cross and north-south line, cannot occur during the display of images in display tubes comprising display windows manufactured in this manner.
It is to be noted that a method of the type described in the opening paragraph is known from U.S. Pat. No. 3,467,059, in which method the part of the provided suspension drained off the display window near its corners is not collected in collection cups but in an annular collection window co-rotating with the display window, from which collection window the suspension is guided to a stationary annular collection basin arranged outside the window. Such a system for draining the excess of suspension is unsatisfactory, particularly at low rotation velocities. The suspension is distributed on the collection window from where it may easily drip onto the display window, particularly when the axis of symmetry oriented transversely to the inner side of the display window is in a horizontal position. There is no risk of dripping in the method according to the invention.
The part of the suspension which is collected in the collection cups can be transported to the central collection cup by means of, for example, pumps. A simpler method is, however, obtained when the part of the suspension collected in the collection cups is drained from these cups to a central pipe through which it is drained to the central collection cup by means of centrifugal forces generated by the rotation.
At low rotation velocities, particularly when the axis of symmetry oriented transversely to the inner side of the display window is in a horizontal position, the gravitational force on the suspension in the central pipe may be larger than the centrifugal force. Consequently, a part of the suspension which is already present in the pipe might flow back into the collection cups. This increases the risk of suspension dripping onto the display window. When the method is being carried out, it is therefore preferably ensured that the part of the suspension collected in the collection cups is drained from these collection cups to the central pipe in such a way that, during the rotation, the suspension cannot flow back into the collection cups. For the same reason, the risk of dripping is further reduced by ensuring that the part of the suspension collected in the collection cups is drained from the central pipe to the central collection cup in such a way that, during the rotation, the suspension cannot flow back into the central pipe.
The invention also relates to a device for providing a layer of a coating material on the inner side of a bowl-shaped rectangular display window of a color display tube, which device comprises a turntable which is rotatable about an axis of rotation oriented transversely to the turntable and tiltable about a tilt axis oriented transversely to the axis of rotation, said turntable having a holder for securing the display window on the turntable in such a way that its axis of symmetry oriented transversely to the inner side coincides with the axis of rotation of the turntable and of four collection cups which are secured on the turntable at the area of the corners of a display window to be arranged on the turntable, so that, during operation of the device, a part of a suspension provided on the inner side of the display window and drained off the display window during the rotation can be collected by said collection cups.
To ensure that the rotation can be performed at a low velocity when coating the inner side of the display window without drops from the collection cups falling on the display window, this device according to the invention is characterized in that the turntable is further provided with a central collection cup and with means for draining the part of the suspension collected in the collection cups near the corners of the display window to the central collection cup during operation of the device.
The part of the suspension collected in the collection cups can be transported to the central collection cup by means of, for example, pumps, but a simpler device is obtained when the means for draining suspension from the collection cups to the central collection cup comprises a hose secured to the turntable, which hose is bent in accordance with a circle having its center on the axis of rotation and being located outside the four collection cups, the hose being circumferentially coupled to the four collection cups and both its ends being coupled to the central collection cup arranged between these two ends.
Both ends of the hose are preferably coupled to the central collection cup by means of two connection pipes intersecting a wall of the central collection cup, which connection pipes are slid into the ends of the hose outside the central collection cup and project through some distance into the wall within the central collection cup. It is thereby prevented that, during the rotation, suspension can flow back from the central collection cup to the hose.
Furthermore, the hose is preferably coupled circumferentially to the four collection cups by means of drain pipes formed on the collection cups and projecting through some distance into the hose whose wall is to this end provided with four apertures narrowly enclosing the drain pipes. It is thereby also prevented that suspension flows back from the hose into the collection cups.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.